Method and apparatus for controlling spiral concentrators



Feb. 15, 1966 L. G. HENDRlcKsoN 3,235,081

METHOD AND APPARATUS FOR CONTROLLING SPIRAL CONCENTRATORS PULP STREAM FROM SP/RAL FEED DIS'- TR/BUTR /lv VEN Tof?. PULP STREAM Lun/ER G, HE/vDH/cxso/v f@ @y ,Ewa/5% SPIRAL FEED Attorney ro sP/RAL Feb. 15, 1966 G. HENDRlcKsoN METHOD AND APPARATUS FOR CONTROLLING SPIRAL CONCENTRATORS 2 Sheets-SheerI 2 Filed July 10, 1962 sPL/TTER A mas/HERE All? SUPPLY INVENTOR. LUTHER G. HENDR/CKSON 5y Emu@ 91m Aforney 3,235,681 MEIHD AND APPARATUS FR CONTROLLING SPERAL CONCENIRATGRS Luther G. Hendrickson, Duluth, Minn., assignor to United States Steel Corporation, a corporation of Delaware Filed .luly 10, 1962, Ser. No. 208,886 8 Claims. (Cl. 209-459) This invention relates to an improved method and apparatus for controlling flow of materials in a spiral concentrating plant.

Conventionally a spiral concentrating plant has a large bank of parallel rougher spirals and a smaller bank of parallel cleaner spirals. Each spiral includes a helical trough and a vertical collecting pipe which extends along the axis of the helix. A pulp of water and nely divided solids (for example minus 50 mesh iron ore) is introduced from a distributor to the upper ends of the rougher spirals. As the pulp Hows down the trough of each spiral, heavier particles among the solids remain nearer the axis of the helix and lighter particles move toward the outside. Wash water is introduced to the pulp stream as it flows down the trough to wash away lighter particles and thus improve the separation. The trough has concentrate collection ports near its inner edge through which heavier particles discharge into the collecting pipe. These ports are equipped with cutters whose position can be adjusted to regulate the width of stream routed to the concentrate. Intermediate and lower density particles discharge separately at the bottom of the trough. Thus rougher spirals separate solids in the pulp into concentrate, middlings and tailings. The rougher concentrate usually goes to the cleaner spirals, which act on it in like manner to produce a nal concentrate. For a more detailed description of a spiral concentrator, reference may be made to printed publications by Taggart, Handbook of Mineral Dressing, copyright 1945, page 11-l35, or Elements of Ore Dressing, copyright 1951, page 212, both published by Iohn Wiley and Sons, Inc., New York.

For spiral concentrators to operate properly, each spiral in a bank should receive a relatively constant weight of solids, and the pulp reaching the spirals should have a relatively constant solids-to-water ratio or density. In practice feed rates often fluctuate. An increase in the feed rate tends to lower the recovery, while a decrease tends to lower the grade of concentrate. Some compensation is .possible by adjusting the wash water rate. For extreme variations it is sometimes necessary to adjust positions of cutters in the concentrate collection ports. This is an extremely time consuming procedure and should be avoided. Adjustment is also possible by manually cutting individual spirals into and out of operation, but the amount of -correction is limited unless a variable speed pump is used.

An object of my invention is to provide an improved control method and apparatus, applicable particularly to` rougher spirals, for automatically maintaining both the weight of solids feeding to each spiral and the solids-towater ratio of the feed at constant values.

A further object is to provide an improved control method and apparatus which utilize a constant speed pump for feeding pulp to a bank of spirals and have automatically adjustable bypasses for cutting individual spirals of the bank into or out of operation as required to maintain relatively constant conditions in spirals remaining in operation.

A further object is to provide an improved method and apparatus for controlling ow of material in a spiral plant ited States Patent O Mice in which automatic adjustments are made both in the volume of water added to an ore pulp and in the proportion of feed bypassing the spirals and returning to the pump to maintain the pulp at a relatively constant den- FIGURE 1 shows a ilowsheet of a spiral concentrating plant which includes a pump box 10, a constant speed pump 12, a spiral feed distributor 13, and a bank of individual parallel spiral concentrators 14, 14a, 145, 14e, ete. If the spirals are used as roughers, the original feed (for example a pulp of water and minus 50 mesh iron ore) commonly is an undcrilow product from a desliming cyclone (not shown). This feed enters the pump box 10, from which pump 12 withdraws it at constant volume and transfers it through a line 15 to the distributor 13. From there lines 16, 16a, etc. carry equal shares of the pulp to the individual spirals, which separate solids therein into concentrate and tailing products. The individual pieces of apparatus and the portion of the owsheet thus far described are conventional and hence not described in greater detail. Although the flowsheet shows only four spirals, a spiral bank in a commercial plant ordinarily has a much larger number (for example eighty to one hundred rougher spirals or about half as many cleaner spirals). In the example of iron ore, proper control can be exercised when about half the number of spirals in a bank operate at all times, while individual spirals among the other half can be cut in or out as needed. The normal rate at which solids feed to the bank should be such that about half the number of controlled spirals (three-quarters of the total) are cut in. Thus the bank readily can handle quantities of feed up to one-third larger or smaller than normal.

As the flowsheet also shows, water is introduced to the pump box 10 through a line 17 to dilute the pulp. As also known in the art, the pump box is equipped with v mechanism for maintaining a substantially constant level of diluted pulp therein. This mechanism includes a level senser and controller 18 connected with the pump box, a valve positioner 19 operatively connected with the controller, and an adjustable valve 20 in the water line 17 leading to the pump box. The valve positioner automatically adjusts the valve to admit water in the volume needed to maintain a constant level in the pump box.

In accordance with my invention, I mount respective funnels 21, 21a, etc. and splitters 22, 22a, etc. in each of the lines 16, 16a, etc. which leads from distributor 13 to the controlled spirals 14, 14a etc. I omit the funnels and splitters from the lines leading to spirals which operate all the time. Each splitter is movable between retracted and extended positions, and FIGURE l shows splitters 22, 22a and 22b retracted and splitter 22C extended. In its retracted or out position, a splitter allows pulp to pass freely through the funnel to the spiral. In its extended or in position, a splitter intercepts the pulp stream and diverts it to a bypass line 23 which leads back to the pump box 10. Thus whenever one of the splitters is in, the share of pulp normally goi-ng to the corresponding spiral returns instead to the pump box, and this spiral is cut out of operation. I connect respective positioners 24, 24a, etc. to the splitters for propelling them between their two positions. I mount a pulp density meter 25 in line 15 which leads from pump 12 to distributor 13. I connect the pulp density meter to a recorder-controller 26, a-nd connect the latter to the positioners 24, 24a, etc. Thus the positioners `automatically extend and retract the splitters to maintain control, as hereinafter explained. The number of pulp shares intercepted Varies inversely With changes in the pulp density.

Splitter FIGURE 2 shows structural details of my preferred form of splitter 22, but it is apparent many mechanical equivalents are possible. The other splitters are similar. The splitter illustrated includes a swinging launder 30 and a pair of links 31 attached to the launder and pivoted to an overhead support 32. The launder has an outlet spout 33. The end of the bypass line 23 carries another funnel 34. The positioner 24 includes a double-acting pneumatic cylinder 35, which contains a reciprocable piston and piston rod 36 connected to launder 313. When the piston rod is retracted, the launder allows the pulp stream to enter funnel 21. When the rod is extended, the launder intercepts the pulp stream, which discharges through spout 33 into funnel 34.

Instruments The individual instruments used in my control apparatus mostly are of conventional construction and available commercially. Hence I have not shown nor described -them in detail, but instead reference can be made to printed publications for showings.

Considine, Process Instruments and Controls Handbook published by McGraw Hill Book Company, copyright 195'7, Library of Congress Catalog Card No. 56- S169, shows and describes instruments suitable for several of my purposes. Considine shows a bubbler (page -23) and a force balance unit (page 5-18) of types suitable for my level senser and controller 18, and also a diaphragm motor (page -32) of a type suitable for my valve positioner 19. A suitable force balance unit and Water valve also are available commercially from Fisher Governor Co., Marshalltown, Iowa, as types 2560-2491) and 657AR respectively. This particular force balance unit gives an output signal which varies from 3 to l5 p.s.i.g and is suihcient to operate the valve motor directly. A printed publication by Fisher Governor Co. entitled Fisher Level Trols, Bulletin F-4A, also contains a schematic showing a-nd a description. A suitable recorder is available commercially from Minneapolis-Honeywell Regulator Co., Minneapolis, Minn. as the Class 14 Recorder equipped with front set mercury switches. These switches provide an error signal when the pulp density is above or below the set point. The controller includes a circuit hereinafter described which utilizes the error signal at spaced intervals to actuate the positioners in sequence; that is, as long as the error persists, the controller periodically actuates one additional positioner, for example every two minutes. The pulp density meter can be an Ohmart cell as shown in Ohmart Patent No. 2,763,790.

C ontrol ler circui t FIGURE 3 shows schematically one form of electric circuit which can be included in the recorder-controller 26 for operating the splitter positioners 24, 24a, etc. at spaced intervals in response to signals from the recorder. The circuit includes two lines 39 and 40 connected to a suitable power source, and a power on and olf switch 41. The recorder has normally open high and low switches 42 and 143, which I adjust to give a dead band of about 0.02 sp. gr. points, and a set point knob (not show-n) for adjusting the position of these switches. For example if I adjust the set point to 1.40 sp. gr., the high switch 42 closes when the pulp density meter 25 shows a sp. gr. 1.41 or higher and the low switch at 1.39 sp. gr. or lower. I connect the coil of a relay A across lines 39 and 40 in series with the high switch 42 and back contact B1 of a relay B. Similarly I connect the coil of relay B across these lines in series with the low switch 43 and a back contact A1 of relay A. Thus relay A picks up when the high switch 42 closes and relay B when the low switch 43 closes. When either of these relays pick up, it locks in via its own front contact AZ or B2 and a front contact C1 of a timer-controlled relay C; its back contact A1 or B1 opens to prevent the other from picking up. As explained hereinafter, relay C also picks up with relay A or B and closes its contact C1, provided a sufficient time has elapsed since the previous operation.

I connect an add coil 44 of a stepping relay across lines 39 and 40 in series with front contacts A3 and C2 of relays A and C and a back contact D1 of a relay D. Thus the add coil is energized when both relays A and C pick up. The stepping relay is of a type available commercially, such as the Guardian RAS- and it includes a series of contacts 46, 46a, 4611, etc. Every time the add coil is energized, the next contact ahead in the series closes and the last contact previously closed opens. I connect solenoid windings 47, 47a, 47b, etc. across lines 39 and 40 in series with contacts 46, 46a, 46h, etc. respectively. Whenever the add coil closes a contact, the corresponding solenoid winding is energized and the armature of this solenoid operates a four-way valve 4S, 48a, 48h, etc. The valve admits compressed air to cylinder 35, 35a, 35h, etc. in a direction to retract the respective splitter 22, 22a, 22b, etc. When the add coil closes a contact of the stepping relay, all solenoids preceding the newly-energized solenoid must remain energized to hold the preceding spirals in operation when the additional spiral goes into operation. To hold solenoid 47 energized when solenoid 47a is energized, I connect the coil of a relay E in parallel with solenoid 47a and I connect a front contact E1 of this relay in parallel with the contact 46. Thus relay E picks up with solenoid 47a and closes its contact E1, whereby solenoid 47 remains energized even though Contact 46 opens. I connect additional relays F, etc. in a similar relation with succeeding solenoids 47b, etc. The coil of the aforementioned relay D is connected in parallel with the last solenoid, illustrated as solenoid 47C. When the last spiral goes into operation and relay D picks up, contact D1 opens and prevents the add coil 44 from being energized further.

The stepping relay also has a substract coil 49 which I connect across lines 39 and 40 in series with front contacts B3 and C3 of relays B and C and a back contact G1 of a relay G. Thus the subtract coil is energized when both relays B and C pick up. Every time the subtract coil is energized, 'the next contact back in the series 46, 46a, 46b, etc. closes and the last contact previously close opens. For example, if the spirals are operating as shown in FIGURE 1 and the subtract coil is energized to talce the next spiral 14b out of operation, contact 46!) opens and 46a closes. Opening of contact 46h deenergizes solenoid 47h and relay F. Deenergizing the solenoid operates valve tb, positioner 35h and splitter 22.6, while deenergizing the relay opens contact F1. Solenoid 47a remains energized, but current is supplied thereto via contact 46a rather than F1 as previously. I connect the coil of relay G across lines 39 and 40 in series with the first contact 4611 of the stepping relay. After the last controlled spiral 47 goes out of operation, the next time the subtract coil is energized contact 4611 closes and relay G picks up. Contact G1 opens and prevents the subtract coil from being energized further.

V timer times out, the circuit is reset.

I connect a timer Si) across lines 39 and 40 in series with a contact TD1 of a thermal time delay relay TD. Both the timer and time delay relay are of types available commercially. A suitable timer Sti is manufactured by Eagle Signal Corporation, Moline, Illinois, and identified as Cycl-Flex Type P23A6. The timer has a contact Sila which closes after the timer is energized and times out for a set interval (two to three minutes) and opens when the timer is deenergized. A suitable time delay relay TD is the Amperite Type 115C-2. When the time delay relay is energized, it commences to heat, and after an interval of several seconds opens its contact TD1. When deenergized, it cools, and after a delay of several seconds closes this contact. I connect the time delay relay TD and the coil of relay C in parallel, and connect one side of each to line 4d and the other side to contact 50a. I connect front contacts A4 and B4 of the respective vrelays in parallel, and connect one side of each to line 39 and the other side to contact 50a.

If the control has not acted for several minutes, timer 50 has timed out and Contact Stia is closed. If either relay A or B picks up, both the time delay relay TD and relay C likewise pick up. Contact C1 closes to lock in relay A or B, and contacts C2 and C3 close to complete the current-path through the add or subtract coils 44 or 4.19, as already explained. The time delay relay TD commences to heat, and after a delay of several seconds, contacts TD1 open. Timer 50 is deenergized and contacts 53a open, whereupon relay C and time delay relay TDdrop out. Contacts C2 and C3 open and deenergizes the ad or subtract coil, which in the meantime has furnished the signal that cuts one spiral into or out of operation. When the time delay relay TD cools, contacts TD1 close and energize timer 50. When Athe If addition or Subtraction of a single spiral is enough to bring the pulp density into the set range, switch d2 or 43 opens and relay A or .B drops out. Otherwise relay A or B remains energized, and the cycle repeats. In either case the minimum interval before a second spiral can be cut into or out of operation is the time required for timer 50 to time out, thus forestalling hunting and erratic operation.v

Preferably the circuit also includes manually operable contacts for cutting spirals into or out of operation when desired. For this purpose I connect a switch Sl between the add coil 44 and line 39 in parallel with contacts A3 and C2, and I connect a switch 52 between the subtract coil 49 andy line 39-in parallel with contacts B3 and C3. -Whenl cl-ose switch 51 or 52, the add or subtract coil is: energized, and Vcuts a spiral into or out of operation-1in the. same vmanner as when energized through the automatic circuit. I also include individual manual and automatid-7 switches S3, 53a, etc. in series with the respective solenoids 4-7, 47a, etc.

Operation To describe the way my control operates, I assume initially the ilow of materials is stabilized with the parts in the positions shown in FIGURE l (splitter 22C in, the others out). Pump 12 continuously withdraws a constant volume of pulp from the pump box It). This pulp is `replaced continuously by an equal volume of pulp made up of (a) new pulp feeding to the pump box, (b) the bypassed pulp share returning to the pump box via line 23, and (c) water introduced via line I7. Now I assume the feed rate of new solids to the pump box diminishes. The level of material in the pump box starts to fall, but the level sensing device 18 detects the fall, whereupon the valve positioner 19 adjusts valve 2t) to admit a greater volume of water to the pump box. The level of material is restored, but the pulp is more dilute. The pulp density meter 25 detects a decrease in density, whereupon the recorder-controller 26 actuates the next splitter positioner 24h to extend splitter 22h. This splitter intercepts the stream of pulp in line 2lb and thus returns a second pulp share to the pump box 10, cutting spiral 14h out of operation. Now the level of material in the pump box commences to rise, whereupon valve 20 is adjusted to admit a smaller volume of water. Additional solids in the second returning pulp share increase the density counter to the effect of water introduced through line I7. If the increase is insufhcient to restore the density to the set point, the foregoing steps are repeated and splitter 22a is extended to return a third pulp share. In this manner the density ultimately returns to its set value, and the flow of materials again is stabilized. The reverse action of course takes placewhen the weight of new solids in the feed increases. Since the pump delivers material at constant volume and the control apparatus maintains a constant density, the weight of solids in each operating spiral necessarily remains constant. In the example of iron ore, the pulp typically has a solids content of about 25 to 40 percent by weight, but for any particular ore the content is held as nearly constant as possible.

From the foregoing description it is seen that my invention affords a dependable automatic method and apparatus for controlling flow of water and solids in a spiral concentrating plant. The invention utilizes only control instruments which are commercially available. Consequently the apparatus is readily assembled and applied to existing concentrating plants. The only special parts needed are my splitters, whose construction I claim also as part of my invention.

While I have shown and described only a single embodiment of my invention, it is apparent that modifications may arise. Therefore, I do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

I claim:

ll. In a concentrating process in which a pulp of Water and finely divided solids feeds into a pump box, additional water is introduced to the pump box to dilute the pulp andthereby decrease its density, the volume of the additional Water is regulated to maintain .a substantially constant level of diluted pulp in the pump box, a pump transfers a constant volume of the diluted pulp in shares to a bank of parallel spiral concentrators, and the concentrators separate solids in the pulp into concentrate and tailings products, the combination therewith of a control method comprising generating a signal representative of the density of the diluted pulp before it reaches the concentrators, cutting individual concentrators in the bank into and out of operation in response to changes in said signal, and returning to said pump box the shares of pulp for concentrators cut out of operation to counter the decrease in density brought about by the additional water, thereby maintaining the pulp in the concentrators at a relatively constant density and maintaining a relatively constant weight of solids in each concentrator in operation.

2. In a concentrating process in which a pulp of water and finely divided solids feeds into a pump box, additional water is introduced to the pump box to dilute the pulp and thereby decrease its density, the volume of the additional vwater is regulated -to maintain a substantially constant level of diluted pulp in the pump box, a pump transfers the diluted pulp at constant volume to a distributor, the pulp goes from the distributor to a bank or" parallel spiral concentrators in equal shares to the individual concentrators, and the concentrators separate solids in the pulp into concentrate and tailing products, the combination therewith of a control method comprising measuring the density of the diluted pulp before it reaches the concentrators, intercepting shares of pulp between said distributor and individual concentrators, varying the number of shares intercepted inversely with density changes, and returning the intercepted shares to the pump box to counter the decrease in density brought about by the additional water, thereby maintaining the pulp in the concentrators at a relatively constant density and maintaining a relatively constant weight of solids in each concentrator in operation.

3. In a concentrating process in which a pulp of water and finely divided solids feeds into a pump box, additional water is introduced to the pump box to dilute the pulp and thereby decrease its density, the volume of the addition water is regulated to maintain a substantially constant level of diluted pulp in the pump box, a pump transfers the diluted pulp at constant volume to a distributor, the pulp goes from the distributor to a bank of parallel spiral concentrators in equal shares to the individual concentrators, and the concentrators separate solids in the pulp into concentrate and tailing products, the combination therewith of a control method comprising measuring the density of the diluted pulp before it reaches the concentrators, intercepting shares of pulp between said distributor and individual concentrators, varying the number of shares intercepted inversely with density changes, bypassing the concentrators to which the inter-cepted pulp shares normally go to cut these concentrators out of operation, and returning7 the intercepted shares to the pump box to counter the decrease in pulp density brought about by the additional water, thereby maintaining the pulp in the concentrators at a relatively constant density and maintaining a relatively constant weight of solids in each concentrator in operation.

4. A method as defined in claim 3 in which the concentrators are roughers and the solids in the feed are an underflow product from a desliming cyclone.

5. In a concentrating plant which includes a pump box, means for feeding a pulp of water and finely divided solids to said pump box, means for introducing additional water to said pump box to dilute the pulp and thereby decrease its density, means regulating the volume of additional water introduced to said pump box to maintain a substantially constant level of diluted pulp therein, a constant speed pump for withdrawing pulp from said pump box, a distributor for dividing pulp transferred thereto into a plurality of equal shares, a bank of parallel spiral concentrators, a line leading from said pump to said distributor, and lines leading from said distributor to the individual concentrators, whereby said pump transfers pulp at constant volume to said distributor and each line leading from said distributor receives an equal share of this pulp, the combination therewith of a control apparatus comprising means for generating a signal representative of the density of the diluted pulp before it reaches Said concentrators, means for intercepting pulp shares between said distributor and said concentrators, means operatively connected with said signal-generating means and with said intercepting means for varying the number of shares intercepted inversely with density changes, and means for returning the intercepted shares to said pump box to counter the decreases in density brought about by the additional water, thereby maintaining the pulp in the concentrators at a relatively constant density and maintaining a relatively constant weight of solids in each concentrator in operation.

6. In a concentrating plant which includes a pump box, means for feeding a pulp of water and finely divided solids to said pump box, means for introducing additional water to said pump box to dilute the pulp and thereby decrease its density, means regulating the volume of additional water introduced to said pump box to maintain a substantially constant level of diluted pulp therein, a constant speed pump for withdrawing pulp from said pump box, a distributor for dividing pulp transferred thereto into a Aplurality of equal shares, a bank of parallel spiral concentrators, a line leading from said pump to said distributor, and lines leading from said distributor to the individual concentrators, whereby said pump transfers pulp at constant volume to said distributor and each line leading from said distributor receives an equal share of this pulp, the combination therewith of a control apparatus comprising means operatively connected with the line leading from said pump for generating a signal representative of the pulp density, a plurality of splitters each in a ditferent line leading from said distributor for intercepting the shares of pulp carried by the respective lines before the pulp reaches said concentrators, means operatively connected with said signal-generating means and with said splitters for varying the number of shares intercepted inversely with density changes, and means for returning the intercepted shares to said pump box to counter the decrease in density brought about by the additional water, thereby maintaining the pulp in the concentrators at a relatively constant density and maintaining a relatively constant weight of solids in each concentrator in operation.

7. In a concentrating plant which includes a pump box, means for feeding a pulp of water and finely divided solids to said pump box, means for introducing additional water to said pump box to dilute the pulp and thereby decrease its density, means regulating the volume of additional water introduced to said pump box to maintain a substantially constant level of diluted pulp therein, a constant speed pump for withdrawing pulp from said pump box, a distributor for dividing pulp transferred thereto into a plurality of equal shares, a bank of parallel spiral concentrators, a line leading from said pump to said distributor, and lines leading from said distributor to the individual concentrators, whereby said pump transfers pulp at constant volume to said distributor and each line leading from said distributor receives an equal share of this pulp, the combination therewith of a control apparatus comprising a pulp density meter operatively connected with the line leading from said pump for generating a signal representative ot' the density of pulp leaving said pump, a plurality of splitters each in a different line leading from said distributor, each of said splitters having a retracted position in which it allows the share of pulp carried by the line to pass to the respective concentrator and an extended position in which it intercepts this pulp share before the pulp reaches the concentrator, means operatively connected with said density meter and with said splitters for shifting the splitters and varying the number of shares intercepted inversely with density changes, and means for returning the intercepted shares to said pump box to counter the decrease in density brought about by the additional water, thereby maintaining the pulp in the concentrators at a relatively constant density and maintaining a relatively constant weight of solids in each concentrator in operation.

8. In a concentrating plant which includes a pump box, means for feeding a pulp of water and finely divided solids to said pump box, means for introducing additional water to said pump box to dilute the pulp and thereby decrease its density, means regulating the volume of additional water introduced to said pump box to maintain a substantially constant level of diluted pulp therein, a constant speed pump for withdrawing pulp from said pump box, a distributor for dividing pulp transferred thereto into a plurality of equal shares, a bank of parallel spiral concentrators, a line leading from said pump to said distributor, and lines leading from said distributor to the individual concentrators, whereby said pump transfers pulp at constant volume to said distributor and each line leading from said distributor receives an equal share of this pulp, the combination therewith of a control apparatus comprising a pulp density meter operatively connected with the line leading from said pump for generating a signal representative of the density of pulp leaving said pump, a plurality of splitters each in a different line leading from said distributor, each of said splitters having a retracted position in which it allows the share of pulp carried by the line to pass to the respective concentrator and an extended position in which it intercepts this pulp share before the pulp reaches the concentrator to cut the concentrator out of operation, positioners operatively connected with said splitters, means operatively connected with said 9 10 density meter and said positioners for actuating the posi- References Cited by the Examiner tionelrs and shifting said splitters in sequence and thus UNITED STATES PATENTS varymg the number of shares intercepted inversely with l density changes, and means for returning the intercepted 2431559 11/194' Humphreys e- 209- 211 shares to said pump box to counter the decrease in density 5 2717078 9/1955 Le 209172-5 brought about by the additional water, thereby main- 21965516 12/1960 Henderson 241-34 taining the pulp in the concentrators ata relatively con- 4 stant density and maintaining a relatively constant Weight HARRY B THORNTON Prlmwy Exammer' of solids in each concentrator in operation. FRANK W. LUTTER, Examiner. 

1. IN A CONCENTRATING PROCESS IN WHICH A PULP OF WATER AND FINELY DIVIDED SOLIDS FEEDS INTO A PUMP BOX, ADDITIONAL WATER IS INTRODUCED TO THE PUMP BOX TO DILUTE THE PULP AND THEREBY DECREASE ITS DENSITY, THE VOLUME OF THE ADDITIONAL WATER IS REGULATED TO MAINTAIN A SUBSTANTIALLY CONSTANT LEVEL OF DILUTED PULP IN THE PUMP BOX, A PUMP TRANSFERS A CONSTANT VOLUME OF THE DILUTED PULP IN SHARES TO A BANK OF PARALLEL SPIRAL CONCENTRATORS, AND THE CONCENTRATORS SEPARATE SOLIDS IN THE PULP INTO CONCENTRATE AND TAILINGS PRODUCTS, THE COMBINATION THEREWITH OF A CONTROL METHOD COMPRISING GENERATING A SIGNAL REPRESENTATIVE OF THE DENSITY OF THE DILUTED PULP BEFORE IT REACHES THE CONCENTRATORS, CUTTING INDIVIDUAL CONCENTRATORS IN THE BANK INTO AND OUT OF OPERATION IN RESPONSE TO CHANGES IN SAID SIGNAL, AND RETURNING TO SAID PUMP BOX THE SHARES OF PULP FOR CONCENTRATORS CUT OUT OF OPERATION TO COUNTER THE DECREASE IN DENSITY BROUGHT ABOUT BY THE ADDITIONAL WATER, THEREBY MAINTAINING THE PULP IN THE CONCENTRATORS AT A RELATIVELY CONSTANT DENSITY AND MAINTAINING A RELATIVELY CONSTANT WEIGHT OF SOLIDS IN EACH CONCENTRATOR IN OPERATION. 